OK, I found a spare volt/fuel gauge here and it's the same as the one in your pic, so I think we have a solid point of reference.
I added descriptions to the wires in your pic trying to show the functions. Important to note that the wire colors here ARE THE WIRE COLORS RIGHT AT THE BACK OF THE GAUGE. The reason that's important to note is that Datsun SOMETIMES CHANGED THE WIRE COLORS AT THE DASH CONNECTORS, and I believe the volt/fuel gauge is one of those situations. So be careful when referencing wire colors on the wiring diagrams since they sometimes changed colors on the gauge side of the connectors.
In other words, the colors on the diagrams are the colors that exist ON THE HARNESS SIDE, and not always on the gauge side.
But all that said... Here's what the wires do right at the back of the gauge:
Most of the connections are unique and should be relatively easy to identify on the dash side (assuming things aren't TOO hacked up), but there are two connections that are not. There are a) two ground connections, and there are b) two "hot in ON and START" connections.
a) The two ground connections just go to ground and there's no real requirement for them to be kept separate. If you wanted to for some reason, you could just connect those two to the exact same ground location.
b) The two "hot in ON and START" connections originally went to two different fuses in the block, but if you get really stuck and just can't deal with it anymore, then it probably doesn't matter much which fuse get's which function. But if you really want to figure out the real-real correct fuses, we can work through that too.
So with the different years and different PO's in there, I think it's going to have to be up to you to find the correct connections on the dash side, but that's what they should be connected to.
Is that enough excruciating detail? Hope that helps!